Re: [PATCH] mm/mlock: skip __mm_populate() for MLOCK_ONFAULT

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Wed, Jul 22, 2026 at 08:51:33PM +0800, Wandun Chen wrote:
> From: Wandun Chen <chenwandun@lixiang.com>
>
> MLOCK_ONFAULT only locks pages on future faults, so there is no need to
> fault in non-present pages during the mlock2/mlockall syscall.
> populate_vma_page_range() already returns immediately for VMAs with
> VM_LOCKONFAULT, which means __mm_populate() just loops over VMAs and
> holds mmap_read_lock without doing useful work.
>
> Skip __mm_populate() when MLOCK_ONFAULT is set to avoid this unnecessary
> work.
>
> Signed-off-by: Wandun Chen <chenwandun@lixiang.com>

Well you take mmap read lock and release it (after having held the write lock),
hardly earth-shattering.

And this has been this way for donkey's years I don't really see why we should
care?

Do you have a workload that's heavily dependent on mlock2(..., MLOCK_ONFAULT) or
mlockall(..., MCL_ONFAULT) as a hot path that is seriously contending the mmap
lock?

I don't love how the VMA_LOCKONFAULT_BIT flag works but I'm not sure adding more
churn and code for the sake of it here is really worth it.

Thanks, Lorenzo

> ---
>  mm/mlock.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/mm/mlock.c b/mm/mlock.c
> index efa6716e4dfb..784bd4bfc3bb 100644
> --- a/mm/mlock.c
> +++ b/mm/mlock.c
> @@ -658,9 +658,11 @@ static __must_check int do_mlock(unsigned long start, size_t len,
>  	if (error)
>  		return error;
>
> -	error = __mm_populate(start, len, 0);
> -	if (error)
> -		return __mlock_posix_error_return(error);
> +	if (!vma_flags_test(flags, VMA_LOCKONFAULT_BIT)) {
> +		error = __mm_populate(start, len, 0);
> +		if (error)
> +			return __mlock_posix_error_return(error);
> +	}
>  	return 0;
>  }
>
> @@ -778,7 +780,7 @@ SYSCALL_DEFINE1(mlockall, int, flags)
>  	    capable(CAP_IPC_LOCK))
>  		ret = apply_mlockall_flags(flags);
>  	mmap_write_unlock(current->mm);
> -	if (!ret && (flags & MCL_CURRENT))
> +	if (!ret && (flags & MCL_CURRENT) && !(flags & MCL_ONFAULT))
>  		mm_populate(0, TASK_SIZE);
>
>  	return ret;
> --
> 2.43.0
>